Three general types of exercise have been recognized, isotonic, isometric and isokinetic. The isotonic exercise involves contraction of the muscles against a fixed resistance or load. As a result of the variable length of lever arm formed by the bone structure in the human body, the forces that must be exerted by the muscles vary while the load remains constant. Therefore, the load must be selected to permit movement during the weakest portion of the body motion and the muscles undergo the strongest contractions only during a short portion of the total movement in the body.
Isometric exercises involves the muscular exertion of portions of the body against a load which is stationary and immobile. While this type of exercise permits the maximum contraction of the muscles employed, the body is prevented from any motion.
Isokinetic exercise resolves the problems noted above with isotonic and isometric exercising. In isokinetic exercising, the muscles of the body exert a force against a load or resistance which is moving at essentially a constant velocity and relatively independent of the actual force exerted by the muscles. Therefore, for rehabilitation and exercise of a portion of the body, such as the ankle and lower leg, a device permitting isokinetic exercises would be most preferable.
The human foot is capable of a wide range of motion provided by the ankle joint and lower leg. The major motion provided by the ankle joint is dorsal and plantar flexion. In plantar flexion, the foot is pivoted about the ankle joint to move the toes downward below the ankle. This is done when a person stands on their toes. In dorsiflexion, the foot is pivoted about the ankle joint to draw the toes upward above the heel.
The ankle joint also permits limited motion in inversion and eversion. In inversion, the soles of both feet would move toward each other if both feet are inverted simultaneously. In eversion, the soles of the feet would move away from each other if both feet were evered simultaneously.
Finally, the foot and ankle joint may be rotated about an axis extending through the ankle joint and heel by the action of the tibia and fibula, the bones forming the lower leg.